ACME Working Group H. Landau
Internet-Draft January 15, 2019
Intended status: Standards Track
Expires: July 19, 2019
CAA Record Extensions for Account URI and ACME Method Binding
draft-ietf-acme-caa-06
Abstract
The CAA DNS record allows a domain to communicate issuance policy to
CAs, but only allows a domain to define policy with CA-level
granularity. However, the CAA specification also provides facilities
for extension to admit more granular, CA-specific policy. This
specification defines two such parameters, one allowing specific
accounts of a CA to be identified by URI and one allowing specific
methods of domain control validation as defined by the ACME protocol
to be required.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Extensions to the CAA Record: accounturi Parameter . . . . . 3
3.1. Use with ACME . . . . . . . . . . . . . . . . . . . . . . 3
3.2. Use without ACME . . . . . . . . . . . . . . . . . . . . 4
4. Extensions to the CAA Record: validationmethods Parameter . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . 4
5.1. Limited to CAs Processing CAA Records . . . . . . . . . . 4
5.2. Restrictions Ineffective without CA Recognition . . . . . 5
5.3. Mandatory Consistency in CA Recognition . . . . . . . . . 5
5.4. URI Ambiguity . . . . . . . . . . . . . . . . . . . . . . 6
5.5. Authorization Freshness . . . . . . . . . . . . . . . . . 6
5.6. Use with and without DNSSEC . . . . . . . . . . . . . . . 7
5.7. Restrictions Supercedable by DNS Delegation . . . . . . . 8
5.8. Misconfiguration Hazards . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Normative References . . . . . . . . . . . . . . . . . . . . 8
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
This specification defines two parameters for the "issue" and
"issuewild" properties of the Certification Authority Authorization
(CAA) DNS resource record [RFC6844]. The first, "accounturi", allows
authorization conferred by a CAA policy to be restricted to specific
accounts of a CA, which are identified by URIs. The second,
"validationmethods", allows the set of validation methods supported
by a CA to validate domain control to be limited to a subset of the
full set of methods which it supports.
2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119
[RFC2119] and indicate requirement levels for compliant ACME-CAA
implementations.
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3. Extensions to the CAA Record: accounturi Parameter
A CAA parameter "accounturi" is defined for the "issue" and
"issuewild" properties defined by [RFC6844]. The value of this
parameter, if specified, MUST be a URI [RFC3986] identifying a
specific CA account.
"CA account" means an object maintained by a specific CA representing
a specific entity, or group of related entities, which may request
the issuance of certificates.
The presence of this parameter constrains the property to which it is
attached. Where a CAA property has an "accounturi" parameter, a CA
MUST only consider that property to authorize issuance in the context
of a given certificate issuance request if the CA recognises the URI
specified as identifying the account making that request.
If a CA finds multiple CAA records pertaining to it (i.e., having
property "issue" or "issuewild" as applicable and a domain that the
CA recognises as its own) with different "accounturi" parameters, the
CA MUST consider the CAA record set to authorize issuance if and only
if at least one of the specified account URIs identifies the account
of the CA by which issuance is requested.
A property without an "accounturi" parameter matches any account. A
property with an invalid or unrecognised "accounturi" parameter is
unsatisfiable. A property with multiple "accounturi" parameters is
unsatisfiable.
The presence of an "accounturi" parameter does not replace or
supercede the need to validate the domain name specified in an
"issue" or "issuewild" record in the manner described in the CAA
specification. CAs MUST still perform such validation. For example,
a CAA "issue" property which specifies a domain name belonging to CA
A and an "accounturi" parameter identifying an account at CA B is
unsatisfiable.
3.1. Use with ACME
An ACME [I-D.ietf-acme-acme] account object MAY be identified by
setting the "accounturi" parameter to the URI of the ACME account
object.
Implementations of this specification which also implement ACME MUST
recognise such URIs.
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3.2. Use without ACME
The "accounturi" specification provides a general mechanism to
identify entities which may request certificate issuance via URIs.
The use of specific kinds of URI may be specified in future RFCs, and
CAs not implementing ACME MAY assign and recognise their own URIs
arbitrarily.
4. Extensions to the CAA Record: validationmethods Parameter
A CAA parameter "validationmethods" is also defined for the "issue"
and "issuewild" properties. The value of this parameter, if
specified, MUST be a comma-separated string of challenge method
names. Each challenge method name MUST be either an ACME challenge
method name or a CA-assigned non-ACME challenge method name.
The presence of this parameter constrains the property to which it is
attached. A CA MUST only consider a property with the
"validationmethods" parameter to authorize issuance where the name of
the challenge method being used is one of the names listed in the
comma-separated list.
Where a CA supports both the "validationmethods" parameter and one or
more non-ACME challenge methods, it MUST assign identifiers to those
methods. If appropriate non-ACME identifiers are not present in the
ACME Validation Methods IANA registry, the CA MUST use identifiers
beginning with the string "ca-", which are defined to have CA-
specific meaning.
5. Security Considerations
This specification describes an extension to the CAA record
specification increasing the granularity at which CAA policy can be
expressed. This allows the set of entities capable of successfully
requesting issuance of certificates for a given domain to be
restricted beyond that which would otherwise be possible, while still
allowing issuance for specific accounts of a CA. This improves the
security of issuance for domains which choose to employ it, when
combined with a CA which implements this specification.
5.1. Limited to CAs Processing CAA Records
All of the security considerations of the CAA specification are
inherited by this document. This specification merely enables a
domain with an existing relationship with a CA to further constrain
that CA in its issuance practices, where that CA implements this
specification. In particular, it provides no additional security
above that provided by use of the unextended CAA specification alone
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as concerns matters relating to any other CA. The capacity of any
other CA to issue certificates for the given domain is completely
unchanged.
As such, a domain which via CAA records authorizes only CAs adopting
this specification, and which constrains its policy by means of this
specification, remains vulnerable to unauthorized issuance by CAs
which do not honour CAA records, or which honour them only on an
advisory basis. Where a domain uses DNSSEC, it also remains
vulnerable to CAs which honour CAA records but which do not validate
CAA records by means of a trusted DNSSEC-validating resolver.
5.2. Restrictions Ineffective without CA Recognition
The CAA parameters specified in this specification rely on their
being recognised by the CA named by an "issue" or "issuewild" CAA
property. As such, the parameters are not an effective means of
control over issuance unless a CA's support for the parameters is
established beforehand.
CAs which implement this specification SHOULD make available
documentation indicating as such, including explicit statements as to
which parameters are supported. Domains configuring CAA records for
a CA MUST NOT assume that the restrictions implied by the
"accounturi" and "validationmethods" parameters are effective in the
absence of explicit indication as such from that CA.
CAs SHOULD also document whether they implement DNSSEC validation for
DNS lookups done for validation purposes, as this affects the
security of the "accounturi" and "validationmethods" parameters.
5.3. Mandatory Consistency in CA Recognition
A CA MUST ensure that its support for the "accounturi" and
"validationmethods" parameters is fully consistent for a given domain
name which a CA recognises as identifying itself in a CAA "issue" or
"issuewild" property. If a CA has multiple issuance systems (for
example, an ACME-based issuance system and a non-ACME based issuance
system, or two different issuance systems resulting from a corporate
merger), it MUST ensure that all issuance systems recognise the same
parameters.
A CA which is unable to do this MAY still implement the parameters by
splitting the CA into two domain names for the purposes of CAA
processing. For example, a CA "example.com" with an ACME-based
issuance system and a non-ACME-based issuance system could recognise
only "acme.example.com" for the former and "example.com" for the
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latter, and then implement support for the "accounturi" and
"validationmethods" parameters for "acme.example.com" only.
A CA which is unable to ensure consistent processing of the
"accounturi" or "validationmethods" parameters for a given CA domain
name as specifiable in CAA "issue" or "issuewild" properties MUST NOT
implement support for these parameters. Failure to do so will result
in an implementation of these parameters which does not provide
effective security.
5.4. URI Ambiguity
Suppose that CA A recognises "a.example.com" as identifying itself,
CA B is a subsidiary of CA A which recognises both "a.example.com"
and "b.example.com" as identifying itself.
Suppose that both CA A and CA B issue account URIs of the form
"account-id:1234"
If the CA domain name in a CAA record is specified as "a.example.com"
then this could be construed as identifying account number 1234 at CA
A or at CA B. These may be different accounts, creating ambiguity.
Thus, CAs MUST ensure that the URIs they recognise as pertaining to a
specific account of that CA are unique within the scope of all domain
names which they recognise as identifying that CA for the purpose of
CAA record validation.
CAs MUST satisfy this requirement by using URIs which include an
authority:
"https://a.example.com/account/1234"
5.5. Authorization Freshness
The CAA specification governs the act of issuance by a CA. In some
cases, a CA may establish authorization for an account to request
certificate issuance for a specific domain separately to the act of
issuance itself. Such authorization may occur substantially prior to
a certificate issuance request. The CAA policy expressed by a domain
may have changed in the meantime, creating the risk that a CA will
issue certificates in a manner inconsistent with the presently
published CAA policy.
CAs SHOULD adopt practices to reduce the risk of such circumstances.
Possible countermeasures include issuing authorizations with very
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limited validity periods, such as an hour, or revalidating the CAA
policy for a domain at certificate issuance time.
5.6. Use with and without DNSSEC
Where a domain chooses to secure its nameservers using DNSSEC, the
authenticity of its DNS data can be assured, providing that a given
CA makes all DNS resolutions via an appropriate, trusted DNSSEC-
validating resolver. A domain can use this property to protect
itself from the threat posed by a global adversary capable of
performing man-in-the-middle attacks, which is not ordinarily
mitigated by the "domain validation" model.
In order to facilitate this, a CA validation process must either rely
solely on information obtained via DNSSEC, or meaningfully bind the
other parts of the validation transaction using material obtained via
DNSSEC.
The CAA parameters described in this specification can be used to
ensure that only validation methods meeting these criteria are used.
In particular, a domain secured via DNSSEC SHOULD either:
1. Use the "accounturi" parameter to ensure that only accounts which
it controls are authorized to obtain certificates, or
2. Exclusively use validation methods which rely solely on
information obtained via DNSSEC, and use the "validationmethods"
parameter to ensure that only such methods are used.
Use of the "accounturi" or "validationmethods" parameters does not
confer additional security against an attacker capable of performing
a man-in-the-middle attack against all validation attempts made by a
given CA which is authorized by CAA where:
1. A domain does not secure its nameservers using DNSSEC, or
2. That CA does not perform CAA validation using a trusted DNSSEC-
validating resolver.
Moreover, use of the "accounturi" or "validationmethods" parameters
does not mitigate against man-in-the-middle attacks against CAs which
do not validate CAA records, or which do not do so using a trusted
DNSSEC-validating resolver, regardless of whether those CAs are
authorized by CAA or not; see Section 5.1.
In these cases, the "accounturi" and "validationmethods" parameters
still provide an effective means of administrative control over
issuance, except where control over DNS is subdelegated (see below).
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5.7. Restrictions Supercedable by DNS Delegation
Because CAA records are located during validation by walking up the
DNS hierarchy until one or more records are found, the use of the
"accounturi" and "validationmethods" parameters, or any CAA policy,
is not an effective way to restrict or control issuance for
subdomains of a domain, where control over those subdomains is
delegated to another party (such as via DNS delegation or by
providing limited access to manage subdomain DNS records).
5.8. Misconfiguration Hazards
Because they express a restrictive security policy, misconfiguration
of the "accounturi" or "validationmethods" parameters may result in
legitimate issuance requests being refused.
6. IANA Considerations
None. As per the CAA specification, the parameter namespace for the
CAA "issue" and "issuewild" properties has CA-defined semantics.
This document merely specifies a RECOMMENDED semantic for parameters
of the names "accounturi" and "validationmethods".
7. Normative References
[I-D.ietf-acme-acme]
Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
Kasten, "Automatic Certificate Management Environment
(ACME)", draft-ietf-acme-acme-18 (work in progress),
December 2018.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
.
[RFC6844] Hallam-Baker, P. and R. Stradling, "DNS Certification
Authority Authorization (CAA) Resource Record", RFC 6844,
DOI 10.17487/RFC6844, January 2013,
.
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Appendix A. Examples
The following shows an example DNS zone file fragment which nominates
two account URIs as authorized to issue certificates for the domain
"example.com". Issuance is restricted to the CA "example.net".
example.com. IN CAA 0 issue "example.net; \
accounturi=https://example.net/account/1234"
example.com. IN CAA 0 issue "example.net; \
accounturi=https://example.net/account/2345"
The following shows a zone file fragment which restricts the ACME
methods which can be used; only ACME methods "dns-01" and "xyz-01"
can be used.
example.com. IN CAA 0 issue "example.net; \
validationmethods=dns-01,xyz-01"
The following shows an equivalent way of expressing the same
restriction:
example.com. IN CAA 0 issue "example.net; validationmethods=dns-01"
example.com. IN CAA 0 issue "example.net; validationmethods=xyz-01"
The following shows a zone file fragment in which one account can be
used to issue with the "dns-01" method and one account can be used to
issue with the "http-01" method.
example.com. IN CAA 0 issue "example.net; \
accounturi=https://example.net/account/1234; \
validationmethods=dns-01"
example.com. IN CAA 0 issue "example.net; \
accounturi=https://example.net/account/2345; \
validationmethods=http-01"
The following shows a zone file fragment in which only ACME method
"dns-01" can be used, but non-ACME methods of issuance are also
allowed.
example.com. IN CAA 0 issue "example.net; \
validationmethods=dns-01,non-acme"
Author's Address
Hugo Landau
Email: hlandau@devever.net
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